Synchrotron-based spectromicroscopic characterization of aerosol chemistry

Abstract

X-ray fluorescence microscopy and near edge X-ray fluorescence spectroscopy were used to determine chemical properties, including the oxidation state and chemical speciation, for both individual aerosol particles and bulk aerosol samples. When used in conjunction with ancillary measurements synchrotron-based techniques can provide evidence of underlying chemical mechanisms occurring during atmospheric transport. Phosphorus and iron, vital nutrients for all organisms, control the primary productivity of vast ocean regions, and aerosol deposition constitutes a major source of nutrients to remote ocean regions. However, the factors controlling the solubility and bioavailability of aerosol iron and phosphorus remain unclear. In Chapter 2, phosphorus from Mediterranean aerosols were examined, and aerosol phosphorus from European air masses was found to be more than three times more soluble than phosphorus from North African air masses. This suggests that while North African air masses deliver more total phosphorus to the Mediterranean, the phosphorus from Europe is substantially more soluble, and therefore bioavailable. Chapter 3 examines the composition and oxidation of sulfur in ambient aerosols collected from the greater Atlanta, Georgia area. Metal sulfates were found at both the bulk and individual particle levels, highlighting the importance of sulfur and iron interactions in the atmosphere. Individual aerosol particles consistently contained reduced sulfur in the elemental sulfur oxidation state. The reduced sulfur, S0, was found to account for up to 20% of total sulfur in large particles (>1 μm). The ultimate source of the reduced sulfur is likely a primary emission, such as incomplete combustion or aerosolized soil bacteria. Chapter 4 focuses on the characterization of iron Saharan dust. Examination of Saharan dust after different distances of atmospheric transport revealed shifts in oxidation state, composition, and pH. The longer air masses spent travelling through the atmosphere, the more reduced the iron became; this suggests that a reductive mechanism is at work in the atmosphere to transform aerosol iron in Saharan dust. The pH was also consistently lower in samples that had a longer travel time. The finding of lower pH in these samples suggests that acidic processes are working to solubilize aerosol iron in Saharan dust. This is further supported by the finding of iron sulfates and iron phosphates, phases that likely formed as a result of secondary acidic processes in the atmosphere. In Chapter 5, a global sample set was compiled. Generally, samples that contained high total iron contained iron that was relatively less soluble, and conversely, samples with low total contained iron that was relatively more soluble. Ambient aerosols across all sampling locations were dominated by iron oxides and often contained iron sulfates as a major constituent. The presence of iron sulfates could indicate that acidic reactions are taking place in the samples, however; there was no unifying factor that appeared to control the solubility of aerosol iron across all sampling locations. There were general trends among specific sampling locations. For example, Mediterranean samples showed iron solubility was correlated with pH, suggesting acidic reactions may control the solubility of aerosol iron at this location. This sampling location specific relationships suggest that source region plays a key role in determining the solubility of aerosol iron.